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Simplifying Thermal Energy Storage while Optimizing Profits

July 22, 2016DavidGeneralComments Off on Simplifying Thermal Energy Storage while Optimizing Profits

Thermal-Energy-Storage-Fig-1

Thermal Energy Storage (TES) systems are key players in increasing gas-turbine-powered plants profitability and improving the reliability of operation on the distribution grid. By using a TES system, plants can exploit daily pricing patterns to chill water in off-peak hours and then provide turbine inlet-air cooling in peak demand periods to boost output and improve heat rate.

Any power plant that utilizes a TES system will be able to quantify the monetary benefits immediately. However, California’s AB 2514 Storage policy is in action and is requiring the California Public Utilities Commission to create a commercially viable energy storage system for the state’s investor-owned utilities (IOUS) and to be in full effect by 2020. Although, this is the first law nationwide that is making it mandatory to have an energy storage system in effect, we may see more plans like this showing up in other states as well.

TES optimization is performed by maximizing the cash flow derived from chilled-water resources while respecting the plants’ physical and operational constraints. Real Time Power’s adaptive and intelligent TES Advisor (TESA) Solution provides an easy and simplified operation schedule for day-ahead and real time bids which takes away the guess work of dispatchers and plant operators.

With TESA, operators and dispatchers are shown the appropriate gas turbine inlet temperature set point as well as the recommended chilled water tank operation mode and are sent directly to the operators’ DCS.

TESA is the missing link to your power plant’s success with thermal energy storage and greatly simplifies a very complex system.

For more information please contact Gina Calanni at 281 971 9756.

Improve plant profitability by optimizing thermal energy storage

October 20, 2015DavidGeneralComments Off on Improve plant profitability by optimizing thermal energy storage

Improve plant profitability by optimizing thermal energy storage

Thermal energy storage (TES) systems increase profitability of gas-turbine-powered generating plants by exploiting daily pricing patterns to chill water in off-peak hours and then provide turbine inlet-air cooling in peak demand periods to boost output and improve heat rate.

However, TES systems often are operated using default running schedules based on vendor design calculations, which do not account for the actual prices of electricity and natural gas, or weather conditions, or the thermodynamic state of the plant. The high variability in external and internal conditions during plant operation implies that a fixed operating policy is sub-optimal in real-world situations.

This means further significant benefits—perhaps $1500 to $3000 per day for 500-750-MW combined cycles—may be available by optimizing TES operation. Real Time Power Inc (RTP) shared with attendees at the recent Combined Cycle Users Group annual meeting (Orlando, Aug 24-27, 2015) the company’s automated software solution for computing the optimal TES run schedule for both day-ahead and real-time markets.

RTP’s David Davis explained the application and how data from both the powerplant and energy trading floor are used in state-of-the-art optimization techniques to maximize cash flow within a given time period.

A typical TES system produces chilled water which can be stored in a large well-insulated tank or sent directly to cooling coils in the gas-turbine air inlet house (Fig 1). Inlet air also can be cooled using chilled water from the tank, a preferred option for peak demand hours to reduce parasitic power consumption.

Thermal Energy Storage Fig 1

The chilled and unchilled water are stored in the same tank; there is little mixing between the two layers. The thermocline level is defined as the height in the tank where there is greatest temperature difference between the warm water above and the cool water below.

TES optimization is performed by maximizing the cash flow derived from the chilled-water resource while respecting all of the physical and operational constraints. Silvia Magrelli, the R&D software engineer at RTP who led development of the application said the optimal schedule requires the “solution of a constrained multivariable nonlinear objective function, and with the selection of the appropriate solver algorithm, the machine-generated answer will provide significant additional revenue gain over and above the default schedule.”

The optimizer is layered on high-accuracy adaptive plant models which allow it to calculate key powerplant operational parameters for the forecasted weather conditions, and then also to predict precisely the incremental power and heat-rate benefit of inlet-air cooling, as well as the auxiliary load of the chiller units.

The final pieces of the jigsaw puzzle are forecasts of electricity and natural-gas prices, which enable decisions regarding GT run schedule. The outcome assures sufficient chilled water is available to meet the forecasted needs of the day-ahead market. Plus, in real-time trading it allows sudden changes in market and/or weather conditions to be assimilated immediately and the best use of the TES re-computed for the remainder of the trading day.

One important benefit of the optimizer, Team RTP stressed: It greatly improves the accuracy of day-ahead load forecasts because it always plans to return the TES tank to a specified state at the end of each day. Thus, at the start of each trading day, the thermocline level is the expected value, and the forecasted load and heat rate can be achieved if the TES is operated according to the optimal schedule.

By contrast, with a default schedule, the end-of-day thermocline level is uncontrolled and will affect megawatt and heat-rate numbers for the following day. Finally, when plant equipment constraints are reached—such as maximum chilled-water flow rate through the cooling coil—the system will plan the day-ahead and real-time schedules based on best achievable performance.

Thermal Energy Storage Fig 2

Real Time Power has tested the application against five years of real price and weather data using actual plant thermodynamic models. Figs 2-4 presents the results of this retrospective comparison for a 3 × 1 combined cycle in the South with a 5.75-million-gal storage tank. The optimizer consistently outperforms the default schedule, and over the five-year period investigated, would have produced a $6.6-million benefit. The saving results from differences between the actual electricity price on the day and price curve used in designing the TES and in compiling the default schedule.

Thermal Energy Storage Fig 3

Installation of the RTP solution involves a server which connects to both the plant DCS and the energy trading desk. Control-room operators have the opportunity to set independent variable values and constraints—gas-turbine availability, for example. The system typically is accessed daily by the trader to produce the day-ahead declaration, and subsequently, as required during the current day, to make changes reflecting the real-time market.

Thermal Energy Storage Fig 4

The optimal solution for a particular running configuration is computed in less than one second, making the system very responsive to changes in market and weather conditions.

Factors which affect Industrial Gas Turbine Air Mass Flow Rate

July 1, 2012DavidGeneralComments Off on Factors which affect Industrial Gas Turbine Air Mass Flow Rate

The main factors we at Real Time Power have observed which affect industrial gas turbine air mass flow rate are:

  1. Shaft rotational speed
  2. Inlet guide vane (IGV) angle
  3. Air density at the compressor inlet
  4. Compressor extraction flow rates

The compressor is well approximated as a constant volumetric flow rate machine when at synchronous speed with fixed IGV angle. The volumetric flow rate is then most influenced by the IGV angle, and the relationship between IGV angle and volumetric flow rate is nonlinear, because the effect of the IGV opening involves either a cosine or sine  function, depending on how the angle is defined. This document will use the convention of IGVs fully open as 0° and fully closed would be -90°. In this case, a cosine curve would be a good fit to the volumetric flow rate response, since cos(-90°) = 0 and cos(0°) = 1. It is often the case that some additional flow rate can be generated by going beyond 0° opening, up to 5-6° is typical. This extra flow is created due to aerodynamic interactions between the IGVs and the first stage of rotating compressor blades, and is not predicted from simple “area of opening” calculations.

With constant volumetric flow rate, mass flow rate will then vary as a function of compressor inlet air density. The main factor which affects air density is the temperature of the air, and this is why evaporative coolers, chillers and foggers are deployed in order to lower the ambient temperature and therefore increase the air density and the air mass flow rate, and ultimately the power output of the gas turbine (which is directly proportional to air mass flow rate). Air pressure and relative humidity also affect air density, and the disadvantage of evaporative coolers and foggers over closed cycle chillers is that the lower density water vapor introduced into the air flow reduces some of the gains of the cooling. The air pressure at the compressor inlet is affected by the ambient air pressure and also by pressure drops across air filters and air conditioning equipment. Fouling of air filters and evaporative cooler media can lead to increased pressure drops and lower gas turbine power output.

The fourth factor listed is compressor extraction flow rates, which typically vary based upon ambient conditions, as well as the condition of the gas turbine. Extracting more flow from the compressor stages will in general increase the mass flow rate into the compressor inlet. When compressor extraction flows are scheduled roughly in proportion to IGV opening, then normalized compressor ratio (compressor discharge pressure / compressor inlet pressure) x (ISO air density / air density) correlates extremely well with the volumetric flow rate measured by Real Time Power’s AirSonic system.

One thing not on the list of major influencers is compressor efficiency (or cleanliness). Only in extreme cases of fouling does the volumetric flow rate decrease. In most normal situations, the effect of compressor fouling is to increase compressor discharge temperature, which will in turn increase turbine exhaust gas temperature and limit the maximum power output of the engine. So the main effect of compressor fouling is on maximum achievable MW output from the gas turbine.